P7: Observing the Universe

Convex / Converging Lenses bring light to a Focus

Power (dioptre) = 1 / Focal Length• More powerful lenses have more curved

surfaces

Simple telescopes have 2 converging lenses. The most powerful one being the eyepiece lens

Magnification = focal length of objective lens / focal length of eyepiece lens

Astronomical objects are so distant that light from them is effectively parallel

• Light on the outside of this picture is close to parallel, whereas in the centre it is more at an angle

Concave Mirror Telescopes• Concave mirrors bring light to a focus• In this telescope, you can think of the concave

mirror as being the “objective lens”• Most astronomical telescopes are this type

Ray Diagrams• Label:

1. Source2. Focal Points3. Real Image4. Principal Axis

• Extended off principal axis

Ray Diagrams• Label:

1. Source2. Focal Points3. Real Image4. Principal Axis

• Extended Source

The larger the lens, the sharper the image

• Telescope must have a larger aperture than the wavelength of radiation detected to produce a sharp image.

• Larger aperture = less diffraction

Telescopes on Earth• Major optical and infrared astronomical

observatories on Earth are mostly in Chile, Hawaii, Australia and the Canary Islands

Astronomical Factors for Telescopes1. High altitudes – Less atmosphere above to

absorb light2. Away from cities – Less light pollution3. Good number of clear nights

Non-Astronomical Factors1. Cost of building observatory2. Environmental impact3. Social Impact4. Working conditions for employees

Telescopes in Space1. Outside Earth’s atmosphere

A. Avoids absorption (Gamma Rays, X-Rays don’t reach Earth’s surface)

B. Avoids refraction of light

2. Very expensive to setup, maintain and repair

3. Uncertainties of Government funding for space programs (EG: Barack Obama has recently cut funding in this area to concentrate on the economy).

International Collaboration• Example: – Gemini Observatory in Chile– Opened 2002– Collaboration between Australia and 6 other countries

Advantages to International Collaboration1. Cost of manufacturing can be shared2. Astronomers from around the world can book time on

telescopes in different countries. This allows them to see stars on other sides of the Earth

3. Pooling of expertise and equipment

Direct or Remote Access Telescopes

• Remote access1. Astronomers don’t need to travel to each

telescope to be able to use it2. Can use telescope at convenient times3. EG: Schools in the UK can access the Royal

Observatory over the internet

Computers and Telescopes

1. Can locate a star and track it across the sky2. Image recorded digitally3. Computer can enhance image (eg: reduce

noise)4. Can share images with other scientists

quickly5. Computers allow hundreds of people from all

over the world to access the same telescope

Parallax• Close stars seem

to move relative to others over the course of the year.

Parallax Angle

• Half the angle moved against a background of distant stars in 6 months.

Parallax Angle Size

• A smaller parallax angle means the star is further away.

Parsecs• A star whose

parallax angle is 1 arcsecond is at a distance of 1 parsec

• Calculate distances in parsecs for simple parallax angles expressed as fractions of a second of arc

Light Year / Parsec• A parsec is similar in magnitude to a light year• 1 Parsec = 3.26163626 Light Years

• Interstellar distances (distance between stars) are a few parsecs (pc)

• Intergalactic distances (distance between galaxies) are measured in megaparsecs (Mpc)

Intrinsic Brightness (Luminosity)– Total Amount of Radiation the Star Gives Out Per Second– Depends on its Temperature and its Size

Observed Brightness• Looking at the night sky, 2 stars may seem to be the same

brightness.• However the intrinsically brighter star may be further away. • If you brought the two stars together so that they were the

same distance from you, one would stand out as being brighter

Cepheid Variables• Cepheid variable pulse in brightness.• Their Period relates to their Brightness.

Working Out Distances Using Cepheid Variables

1. Measure the Period2. Use the Period to work out Intrinsic Brightness3. Measure the Observed Brightness4. Compare the Observed Brightness with the

Intrinsic Brightness to get the Distance

Discovery of Other Stars and Galaxies• Telescopes: Revealed that the Milky Way consists of many

stars and led to the realisation that the Sun was a star in the Milky Way galaxy. Also revealed the existance of “fuzzy” objects which originally were named nebulae.

• Curtis v Shapely Debate: Were Nebulae objects within the Milky Way galaxy or separate galaxies outside it?

• Hubble: Observed Cepheid Variables in one nebula which indicated that it was much further away than any star in the Milky Way, and hence, this nebula was a completely separate galaxy.

Solar v Sidereal Day

• Sidereal Day 23hrs56mins

• Solar Day 24 hours

Different stars are seen at different times of the year

• Planets move in complicated patterns relative to the “fixed” stars

Describing the Position of a Star• 2 angles form Earth are needed:– Angle from North to the Star.– Angle from the Horizon to the Star.

• Solar Eclipse: Sun blocked out. Rare because the Moon’s orbit is tilted 5 degrees.

• Lunar Eclipse: Moon blocked out. More common because the Earth’s shadow is so big.

• Sun, Stars, Moon (and Planets mostly) move across the sky from East to West. IE: everything sets in the West, not just the Sun. This is explained by the Earth’s rotation.

• Sun: 24 hours• Stars: 23 hours 56 minutes• Moon: 25 hours– The Moon takes 28 days to orbit the Earth

completely. It also orbits the Earth from West.

Sun, Stars and Moon

Hot Objects• All hot objects emit a continuous range of

electromagnetic radiation• The greater the Peak Frequency (measured in Hz)

the higher the temperature and intrinsic brightness.• Which is why hot blue objects (high frequency) are

hotter than a hot red objects (low frequency)

Ionisation

• Ionisation is the removal of an electron from an atom.

• Electrons can also move between electron shells within an atom

• This produces line spectra• Each element has a unique line spectra

Electrons move within Atoms

Star Spectrum• Star spectra contain specific spectral lines. These

provide evidence of the elements in the star

Rutherford-Geiger-Marsden Experiment

Describing the Experiment• Expected Results: alpha particles passing through

the plum pudding model of the atom undisturbed.• Observed Results: a small portion of the particles

were deflected, indicating a small, concentrated positive charge (the nucleus).

The Model of the Atom

Past → Present

Structure of the Atom

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The Source of the Sun’s Energy• Up until the mid 19th Century (1850) it was commonly

believed that the Sun was composed of some special material that had the ability to shine eternally.

• Advancements in the true structure of the atom led to the source of the Sun’s energy.

• If you could somehow force the protons present in the nuclei of hydrogen together to form helium nuclei, this would release energy as light and heat.

Nebula• Nebula are clouds of dust, hydrogen and helium.• These materials "clump" together to form larger

clumps. • More mass = more gravity = more mass attracted

Protostar • Proto = Prefix meaning “first”

Compressed Gases

• Increased pressure• Particles closer together• More collisions with other

particles• Friction and collisions

between particles increases the temperature

Decreased VolumeIncreased PressureIncreased Temperature

Absolute Zero• The lowest possible

temperature

• All particles/atoms stop moving completely

• Gas would exert no pressure

Two Competing Forces in the Formation of Protostars

• Pressure of Compressed Gases

• Gravity between the particles of gas

Two Competing Forces• Something is holding these smurfs together

even though they are trying to pull away from each other

Strong Nuclear Force

• Something is holding this Nucleus together even though the protons want to pull away from each other.

Nuclear Fusion• Hydrogen fusing together to make Helium

atoms. This releases lots of energy.

Structure of the Sun

1. Core – Fusion takes place2. Radiative Zone – Energy

transported towards the surface by photons

3. Convective Zone – Energy transported to surface by convection

4. Photosphere – Energy is radiated into space

When Hydrogen runs out Stars start to change.

• Average size stars (like out Sun) turn into Red Giants

• Massive stars turn into Red Supergiants.

Red Giants and Red Super Giants• Higher elements are

made by just fusing more Helium nuclei together: – Carbon – Oxygen– Nitrogen

• This Fusion process also releases energy– What elements are living

organisms made from?

• What element has been made in this diagram?

Red Giants• Red giants lack the mass to compress the core

further at the end of helium fusion. • They then shrink into hot white dwarfs, which

gradually cool.

Red Super Giants• Fusion

Continues in Red Supergiants

• Due to the high pressure in their core larger elements are formed

Red Super Giants

• Fusion stops in Red Super Giants when the core is mostly Iron (Fe)

Supernova Exlosion• After the core is mostly iron, and

fusions stops, a Supernova explosion occurs which ejects much of the star’s materials

Neutron Stars and Black Holes• After a Supernova Explosion, the remaining

material turns into a Black Hole or Neutron Star• Both of these are extremely dense objects